Простой вопрос консольного калькулятора.

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Я довольно новичок в c++, я начал с него примерно два дня назад. Я закодировал полностью функциональный калькулятор dos. Теперь я признаю, что это может быть не самый лучший способ сделать калькулятор, пожалуйста, будьте добры. Вот код. Мой вопрос касается последнего комментария внизу. (Visual Studio 2017)

#include "stdafx.h"
#include <iostream>

//Calculator's adding function.
int addNumbers(int x, int y)
{
	int answer = x + y;
	return answer;
}

//Calc's subtracting function.
int subtractNumbers(int x, int y)
{
	int answer = x - y;
	return answer;
}

//Calc's multiplication function.
int multiplyNumbers(int x, int y)
{
	int answer = x * y;
	return answer;
}

//Calc's division function.
int divideNumbers(int x, int y)
{
	int answer = x / y;
	return answer;
}



int main()
{
	//Requests first number
	std::cout << "Please enter the first number:" << std::endl;
	int number1;
	std::cin >> number1;

	//Second number
	std::cout << "Please enter the second number: " << std::endl;
	int number2;
	std::cin >> number2;

	//Requests an arithmatec operation, and defines it as "operation" so we can call the calculator's functions to it.
	std::cout << "Now, please enter an operator." << std::endl;
	std::cout << " 1 is (+) " << std::endl;
	std::cout << " 2 is (-) " << std::endl;
	std::cout << " 3 is (*) " << std::endl;
	std::cout << " 4 is (/) " << std::endl;
	int operation;
	std::cin >> operation;

	//Calls the calculator's functions to the inputted "operation" variable. This calculates the respective equation.
	if (operation == 1)
		std::cout << "The sum of the two numbers is: " << addNumbers(number1, number2) << std::endl;
	if (operation == 2)
		std::cout << "The difference of the two numbers is: " << subtractNumbers(number1, number2) << std::endl;
	if (operation == 3)
		std::cout << "The product of the two numbers is: " << multiplyNumbers(number1, number2) << std::endl;
	if (operation == 4)
		std::cout << "The quotient of the two numbers is: " << divideNumbers(number1, number2) << std::endl;

	//Error message if input is impossible.
	if (operation > 4)
		std::cout << "Operator error: Please enter a number between 1 and 4." << std::endl;
	if (operation < 1)
		std::cout << "Operator error : Please enter a number between 1 and 4." << std::endl;
		return 0;
}	// (statement that will show error message if anything but 1, 2, 3, 4 is inputted.)

//The last comment there is what I'm trying to accomplish.
//ALSO: How to make it continuous, so when you calculate one thing, the console doesn't close

Что я уже пробовал:

if (operation != 1, 2, 3, 4)
    std::cout << "Eroror";

and

else (operation =! 1, 2, 3, 4)
		std::cout << "error..." << std::endl;

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